1. Field of the Invention
The present invention relates to a toner for developing statically charged images and a process for the preparation of this toner.
2. Description of the Related Art
In the field of the electrophotography, a toner is used for visualizing a statically charged image. This toner consists of particles having a particle size arranged within a certain range, for example, a range of from 5 to 30 .mu.m, which are formed of a composition comprising a resin medium, a colorant, a charge-controlling agent and other additive. A resin having desired electroscopic property and binding property, for example, a styrene resin, is used as the resin medium. Carbon black and other organic or inorganic coloring pigments are used as the colorant.
According to a most typical process for the preparation of a toner for the electrophotography, a resin medium as mentioned above is melt-kneaded with a colorant, the melt-kneaded mixture is cooled and pulverized, and the pulverized product is classified to obtain particles having a particle size included within a certain range. However, the yield of the toner obtained through the pulverization and classification steps is very low and large equipments are necessary for these operations. Accordingly, the manufacturing cost of the toner is very high. Moreover, since shapes of the obtained particles are irregular, the flowability of the toner is generally low and blocking is readily caused.
As the means for preparing fine particles of a colored polymer directly without performing the pulverization, there have been proposed polymerization processes as disclosed in Japanese Patent Publications No. 10231/61, No. 14895/76, No. 17735/78, No. 17736/76, No. 17737/78 and No. 51830/72. These processes are so-called suspension polymerization processes. Namely, a polymerizable composition comprising a polymerizable monomer, a polymerization initiator and a colorant is suspended in an aqueous dispersion medium and is polymerized, whereby a toner is directly prepared. This suspension polymerization process is advantageous in that the formed toner particles have a spherical shape and are excellent in the flowability, the preparation process is simple and the manufacturing cost is low.
However, in the case where a charge-controlling agent is incorporated into the polymerizable composition for obtaining a toner having frictional charge characteristics suitable for the development in the production of a toner by the suspension polymerization, since the charge-controlling agent is hardly distributed on the surfaces of particles of suspended oil drops but are buried in the interior of the particles, in order to obtain a toner having a satisfactory charge quantity, a considerable amount of the charge-controlling agent should be incorporated in the polymerizable composition. However, most of charge-controlling agents exhibit a polymerization-inhibiting action because of the presence of polar groups in the charge-controlling agents, and therefore, the molecular weight of the obtained polymer is low or the polymerization reaction is not completed. Accordingly, the amount used of the charge-controlling agent is limited and a toner satisfactory in the charge characteristics and binding property cannot be obtained.
As the means for overcoming the above-mentioned disadvantage, Japanese Patent Application Laid-Open Specification No. 144836/83 proposes a process for the preparation of a toner, in which a copolymer comprising, as a constituent, a monomer having a polar group, such as acrylonitrile, is incoporated into a polymerizable composition and the suspension polymerization of the polymerizable composition is carried out. In this process, however, in order to obtain a toner having a satisfactory charge quantity, a large quantity of the copolymer should be incorporated into the polymerizable composition, and with increase of the amount incorporated of the copolymer, the viscosity of the polymerizable composition increases, and the oil drop particles come to show a broad particle size distribution when suspended oil drops are formed. Therefore, in the formed toner, a toner particle size applicable to the practical use is not obtained and hence, the classification step becomes necessary, with the result that the process steps become complicated.